on a york-hull ride, the train up was "full" of bikes and a chap appeardd with a complete bike, no bag, with the wheels in an Ikea bag over his shoulder. He put the bike somewhere random in the guards van and the guard was perfectly happy, and the chap then carried the Ikea bag containing two filthy bike wheels into the carriage and put them above his head in the luggage rack.
I'm a fan of a roll of clingfilm for wrapping bikes. Also of supermarket cardboard boxes plus parcel tape for constructing a "bike box":
I've also used a plastic bag that came with a new mattress for the bike when flying, and
@ianmac62 will recall we used plastic bin liners and parcel tape ("tapas adhesivas para parcelas" acccording to Google Translate) to make flimsy bags to get the bikes on a long-distance Spanish coach. I once used my sewing machine to make a pertex bag to get the bike on eurostar to paris and back, so i could ride the Loire valley, but TBH it is easier to cobble something together from plastic and then throw it away.
but you need to be careful of the deraillieur, and especially the hanger, if the bike is dropped on that and it breaks you need a new bike. And i recall
@Trickedem was going to use a "wheels off and bike in plastic bag" system to get to York for the Hull ride, once. So it can, and has been, done.